Pythium Root and Stem Rot -- Pythium ultimum, P. irregulare, Pl aphanidermatum
Host: Broad range of annuals and perennials, including African violet, azelea, geranium, lisianthus, petunia, poinsettia, pothos, vinca, spathiphyllum and gerbera
Symptoms: Often, the first symptoms are lower leaf yellowing, wilting and stunting that mimics nutrient deficiency. Infected roots appear gray, brown or black instead of white, starting at the root tip and spreading throughout the root system, sometimes causing a base canker.
Conditions Favoring Disease: The most common causes of this disease are excess water, either from over watering or poor drainage, and cool soil temperatures. Plants whose roots have been injured by over-fertilization or pests are especially susceptible to infection.
How Pathogen Survives/Disperses: Generally soil born organisms, some Pythiums form swimming spore stages that are favored by subirrigation systems where the spores are easily transferred between plants.
Photo provided courtesy of Chase Research Gardens, write up by GPN Staff.